Pressable touchsurfaces (touch surfaces which can be pressed) are widely used in a variety of input devices, including as the surfaces of keys or buttons for keypads or keyboards, and as the surfaces of touch pads or touch screens. It is desirable to improve the usability of these input systems.
FIG. 2 shows a graph 200 of an example tactile response curve associated with the “snapover” haptic response found in many keys enabled with metal snap domes or rubber domes. Specifically, graph 200 relates force applied to the user by a touchsurface of the key and the amount of key displacement (movement relative to its unpressed position). The force applied to the user may be a total force or the portion of the total force along a particular direction such as the positive or negative press direction. Similarly, the amount of key displacement may be a total amount of key travel or the portion along a particular direction such as the positive or negative press direction.
The force curve 210 shows four key press states 212, 214, 216, 218 symbolized with depictions of four rubber domes at varying amounts of key displacement. The key is in the “unpressed” state 212 when no press force is applied to the key and the key is in the unpressed position (i.e. “ready” position). In response to press input, the key initially responds with some key displacement and increasing reaction force applied to the user. The reaction force increases with the amount of key displacement until it reaches a local maximum “peak force” F1 in the “peak” state 214. In the peak state 214, the metal snap dome is about to snap or the rubber dome is about to collapse. The key is in the “contact” state 216 when the keycap, snap dome or rubber dome, or other key component moved with the keycap makes initial physical contact with the base of the key (or a component attached to the base) with the local minimum “contact force” F2. The key is in the “bottom” state 218 when the key has travelled past the “contact” state and is mechanically bottoming out, such as by compressing the rubber dome in keys enabled by rubber domes.
A snapover response is defined by the shape of the reaction force curve—affected by variables such as the rate of change, where it peaks and troughs, and the associated magnitudes. The difference between the peak force F1 and the contact force F2 can be termed the “snap.” The “snap ratio” can be determined as (F1−F2)/F1 (or as 100*(F1−F2)/F1, if a percent-type measure is desired).